In the recent past, IEEE 802.11 wireless LAN (WLAN) has emerged as prevailing wireless technology throughout the world. The development of 802.11a introduced Orthogonal Frequency Division Multiplexing (OFDM) to 802.11. Subsequently, the 802.11 working group developed the 802.11g amendment, which incorporates the 802.11a OFDM PHY in the 2.4 GHz band. With the development of the IEEE 802.11n, data rate is increased to 300 Mbps in 20 MHz and 600 Mbps in 40 MHz bandwidth. The upcoming IEEE 802.11ac (Very High Throughput (VHT) standard) enhances the data rate beyond 1 Gbps in 5 GHz band.
These standards have used OFDM modulation as the transmission technique for high data rates. The OFDM is a method of transmitting digital data on multiple carrier frequencies. The carrier frequencies are chosen such that they are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both transmitter and receiver. The orthogonality allows for efficient OFDM modulator and OFDM demodulator implementation using the Discrete Fourier Transform (DFT) algorithm on the receiver side, and Inverse DFT (IDFT) on the transmitter side. In OFDM systems, IDFT/DFT is implemented using a serial Fast Fourier Transform (FFT) algorithm for hardware simplicity, which takes input samples serially from 0th sample to (N−1)th samples.
Generally, the implementation of IFFT/FFT is such that it takes samples indexed from 0 to N−1 and provides outputs indexed from 0 to N−1. For IEEE 802.11 standards, in the IFFT equation, the samples are indexed from −N/2 to N/2−1 corresponding to the subcarriers −N/2 to N/2−1. To implement the IFFT, it is suggested in the standard that the samples corresponding to subcarriers indexed from −N/2 to N/2−1 are moved after the samples corresponding to subcarriers indexed from 0 to N/2−1 before sending them to the IFFT in the transmitter. At the receiver, the carriers are required to be remapped before processing by a soft constellation de-mapper. To perform these operations, buffering of samples corresponding to half OFDM symbol is required at both the transmitter and the receiver, which requires a huge amount of memory especially when N is large. It also adds a huge amount of latency in the transceiver path.
Thus, there exists a strong need of a low latency technique for transmission and processing of OFDM symbols.